1. Field of the Invention
This invention relates to scanning probe microscopy and, has application to specific types of scanning probe microscopes, such as the atomic force microscope. More particularly, the present invention relates to a method and apparatus of magnetic force control for scanning probe microscopy. Also, the invention has broader applications, such as in the field of nanolithography.
2. Discussion of Background
Atomic force microscopes are typically devices which have a relatively sharp tip and use low forces to profile a sample surface down to atomic dimensions. Often, atomic force microscopes have a tip on a flexible lever with the vertical position of the tip of the probe detected by a detector. Various detectors have been utilized, such as tunneling tips, optical interferometers and optical beam deflection. Also, various capacitive and inductive proximity detectors are known.
In operation, the atomic force microscope typically will scan the tip of the probe over the sample while keeping the force of the tip on the surface constant, such as by moving either the base of the lever or the sample upward or downward to maintain deflection of the lever portion of the probe constant. Therefore, the topography of a sample may be obtained from data on such vertical motion to construct three-dimensional images of the surface topography. It is also known that atomic force microscopes utilize analog and digital feedback circuits to vary the height of the tip of the probe or the sample based upon the deflection of the lever portion of the probe as an input. As to atomic force microscopes, see also for example, U.S. Pat. No. 5,025,658 and 5,224,376.
The contrast in an atomic force microscope image is due to spatial variations in the force between a probe and the surface. The most common mode of operation uses the short-ranged, Pauli exclusion ("hard-wall") force that arises when the electron clouds of atoms in the tip overlap those in the sample. It is the extremely high gradients of this force which give rise to the excellent resolution of the microscope. However, there are at least three disadvantages to operating in this mode. The downward forces on the entire tip (e.g. van der Waals, capillary forces and tracking forces) must be opposed by upward forces on the few atoms in contact with the surface. The large stresses generated can deform the tip and sample surface thus reducing lateral resolution. Another disadvantage is that when the tip slides in contact with the sample, there are additional lateral (frictional) forces that sometimes cannot be distinguished from normal forces. These lateral forces may also cause damage to the tip and sample. Finally, since the distance dependence of the "hard-wall" force is too steep to be resolved, the contrast is generated almost entirely by topography and, therefore, chemical or material information is unavailable.
Non-contact imaging has been used to circumvent these disadvantages. The forces that generate contrast in this mode are usually attractive (for example van der Waals, electrostatic, and magnetic). If the gradients in these forces exceed the stiffness of the cantilever, a mechanical instability will occur and the tip will snap to the surface. Thus, there is a range of tip-sample separations that are mechanically unstable and, therefore, unsuitable for imaging. For this reason, non-contact imaging is usually performed at large (&gt;10 nm) tip-sample separations. However, resolution increases with decreasing tip-sample separation, so imaging at a small tip-sample separation would be desirable if stability could be achieved. Stability can be achieved by the application of an externally controllable force on the tip and the use of feedback. Externally controllable forces have been applied in atomic force microscopes operating in air using electrostatic forces (S. A. Joyce and J. E. Houston, Rev. Sci. Instr., 62, 710 (1991); G. I. Miller, J. E. Griffith and F. R. Wagner, Rev. Sci., Instrum., 62, 705 (1991); and D. A. Grigg, P. E. Russell and J. E. Griffith, Ultramicroscopy, 42-44, 1504 (1992)), thermal stresses (J. Mertz, O. Marti and J. Mlynek, Appl. Phys. Lett., 62, 2344 (1993)), magnetic force gradients (B. Gauthier-Manuel, Europhys. Lett., 17, 195 (1992)) and inertial forces (P. J. Bryant, H. S. Kim, R. H. Deeken and Y. C. Cheng, J. Vac. Sci. Technol., A 8, 3502 (1990)).
The importance of minimizing imaging forces to decrease sample damage and to improve resolution in contact-mode atomic force microscopy is well known. In non-contact atomic force microscopy better force resolution also means increased spatial resolution. Since the force resolution is set by the smallest deflection of the cantilever detectable, weaker cantilevers typically mean lower imaging forces and are therefore desirable. However, on the other hand, there are many applications where large forces or spring stiffness are advantages or perhaps even necessary. Although larger forces and spring constants can be achieved with stiffer cantilevers, this almost always leads to decreased force resolution and higher imaging forces. It is known that a cantilever may experience a mechanical instability and may snap to the surface when the attractive force gradient exceeds the spring constant. Although it is known to use stiffer cantilevers to allow surface forces to be measured at closer surface separations, such use of stiffer cantilevers also decreases the force resolution.
Another apparatus known in the prior art is the surface force apparatus which measures the force between two surfaces. However, with the surface force apparatus, the length scale is much larger than with the scanning probe microscope. Typically, for example, the radii of curvature of the surfaces are 2 cm which may limit the choice of samples. Further, with the surface force apparatus, the apparatus does not have imaging capabilities.
Also, it is known to use magnetic field gradients to generate magnetic forces to act on a magnetic moment rather than torques on a moment. See, for example, "Use of magnetic forces to control distance in surface force apparatus", Stewart et al, Meas. Sci. Techno. 1, pages 1301-1303 (1990); and "Direct Measurement of the Short-Range Interaction between a Tungsten Tip and a Mica Surface", B. Gauthier-Manuel, Europhysics Letters, 17, pages 195-200, (1992, published in December, 1991).
FIG. 6A is a schematic illustration of the conventional application of a magnetic field gradient to produce a force F acting on a magnetic moment m. In FIG. 6A, a magnetic moment m is oriented parallel to the direction of the magnetic field gradient. While a force is experienced that is proportional to the field gradient, there is no force in a constant field.